In a typical xerographic device, such as a printer, a photoconductor, such as photoreceptor belt (PB), that receives the image is displaced, or revolved, in a process direction. Photoconductors become conductive when exposed to light and are insulative when in the dark. Initially, the photoconductor has a uniform surface potential associated with it. The next step is to expose the image of a document onto the surface of the photoconductor. The imaged areas of the photoconductor remain unexposed and charged, while the areas that receive light become neutralized. The neutralized areas are due to the fact that the charge is drained away from the surface of the photoconductor to a metalized ground. What remains is the electrostatic latent image. This electrostatic image can then be developed. A typical photoconductor layer is usually around 10-50 1m and is coated onto a ground plane. Photoreceptors are typically made into webs or seamless PBs.
Photoconductors typically exhibit basic properties. The electrical conductivity of a photoconductor in the dark must be that of a good insulator. Good insulating qualities are important so that charge patterns can be retained for a period of time long enough to complete the development process. The material must also become electrically conducting during exposure to light. In this way an electrostatic image pattern can be formed on its surface by either optical or laser means. The photoconductor should be fairly strong and be able to with stand continuous charging, discharging by light energy, cleaning, and recharging in the dark. Photoconductors also have dark current associated with them. That is, even when a photoconductor has been charged and is shielded from any source of light, there is still leakage current that flows. The dark current that flows is a result of thermal activity in the photogenerating medium. This decay rate must be accounted for and is kept within certain constraints.
As noted above, in a typical xerographic device, such as a printer, a PB is displaced, or revolved, in a process direction and steered in lateral direction, orthogonal to the process direction, to maintain stable motion and lateral registration (stable orientation and alignment in the lateral direction), in order to minimize color-to-color registration error. The edge of a PB is not perfectly straight due to tolerances involved in the fabrication of the PB. Therefore, upon initial installation of a PB in a xerographic device, contact-type edge sensors detect a shape of the PB edge and data regarding the shape of the edge is used to steer the PB as the PB revolves in the process direction. That is, the shape of the PB edge is “learned” via the contact-type sensors, and any steering of the PB is performed according to the learned shape.
However, as a PB is used, the shape of the edge changes due to wear, edge curl, and other factors. Thus, the learned edge, which is being used to steer the PB, no longer correlates to the actual edge, causing lateral misregistration. If the lateral misregistration exceeds a threshold, a fault is registered by the xerographic device and/or the output of the xerographic device becomes unacceptable. At this point, customer server personnel, engineering support personnel, or field support personnel must be engaged to identify and resolve the problem. Unacceptable performance of the device results in user dissatisfaction and undesirable downtime, while engagement of the personnel noted above results in undesirable downtime as well as undesirable costs for the user or the supplier of the xerographic device.
It is known to provide image registration systems for the correct and accurate alignment, relative to one another, on both axes, of different plural color images on an initial imaging bearing surface member such as (but not limited to) a PB of a xerographic color printer. That is, to improve the registration accuracy of such plural color images relative to one another and/or to the image bearing member
In particular, it is known to provide such imaging registration systems by means of marks-on-belt (MOB) systems, in which edge areas of the image bearing belt laterally within or outside of its normal imaging area are marked with registration positional marks, detectable by an optical sensor. Typically, the MOB system uses distinctive marks, such as “chevron” shaped registration marks, imaged with, and adjacent to, the respective image, and developed with the same toner or other developer material as is being used to develop the associated image, in positions corresponding to, but outside of, the image position, such as putting the marks along the side of the image position or in the inter-image zone between the images for two consecutive prints. Such MOB image position or registration indicia, are thus typically repeatedly developed and erased in each rotation of the photoreceptor belt.